Deep mixing at variable speed in stars: is thermohaline di usion su cient? (original) (raw)
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Deep mixing at variable speed in stars: is thermohaline diffusion sufficient?
2010
We study the circulation of matter in red giants above the H-burning shell, which is known to yield the appearance at the stellar surface of p-capture isotopes like 7Li, 13C, 17O and the unstable 26Al. These isotopes were observed (either in presolar grains of circumstellar origin or in the photospheres of evolved stars) to display abundance ratios to other nuclei that cannot be accounted for by canonical stellar models. Slow mixing below the convective envelope is the usual explanation invoked for their abundance. Diffusion generated by an inversion in the molecular weight mu is today the most commonly assumed driving mechanism for it. We argue that slow transport reaching moderate temperatures (T < 4×107 K), like the one achievable by diffusive processes induced by a mu inversion can account for some, but not all the observational constraints. In particular the production of Li after the first dredge up and of 26Al in the final evolutionary stages both call for substantially di...
Nucleosynthesis of Light-Element Isotopes in Evolved Stars Experiencing Extended Mixing
Publications of the Astronomical Society of Australia, 2009
We present computations of nucleosynthesis in red giants and Asymptotic Giant Branch (AGB) stars of Population I experiencing extended mixing. The assumed physical cause for mass transport is the buoyancy of magnetized structures, according to recent suggestions. The peculiar property of such a mechanism is to allow for both fast and slow mixing phenomena, as required for reproducing the spread in Li abundances displayed by red giants and as discussed in an accompanying paper. We explore here the effects of this kind of mass transport on CNO and intermediate-mass nuclei and compare the results with the available evidence from evolved red giants and from the isotopic composition of presolar grains of AGB origin. It is found that a good general accord exists between predictions and measurements; in this framework we also show which type of observational data best constrains the various parameters. We conclude that magnetic buoyancy, allowing for mixing at rather different speeds, can ...
Atomic diffusion and mixing in old stars
Astronomy & Astrophysics, 2008
Context. Evolutionary trends in the surface abundances of heavier elements have recently been identified in the globular cluster NGC 6397 ([Fe/H] = −2), indicating the operation of atomic diffusion in these stars. Such trends constitute important constraints for the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Aims. We perform an independent check of the reality and size of abundance variations within this metal-poor globular cluster. Methods. Observational data covering a large stellar sample, located between the cluster turn-off point and the base of the red giant branch, are homogeneously analysed. The spectroscopic data were obtained with the medium-high resolution spectrograph FLAMES/GIRAFFE on VLT-UT2 (R ∼ 27 000). We derive independent effective-temperature scales from profile fitting of Balmer lines and by applying colour-T eff calibrations to Strömgren uvby and broad-band BV I photometry. An automated spectral analysis code is used together with a grid of MARCS model atmospheres to derive stellar surface abundances of Mg, Ca, Ti, and Fe. Results. We identify systematically higher iron abundances for more evolved stars. The turn-off point stars are found to have 0.13 dex lower surface abundances of iron compared to the coolest, most evolved stars in our sample. There is a strong indication of a similar trend in magnesium, whereas calcium and titanium abundances are more homogeneous. Within reasonable error limits, the obtained abundance trends are in agreement with the predictions of stellar structure models including diffusive processes (sedimentation, levitation), if additional turbulent mixing below the outer convection zone is included.
Compulsory Deep Mixing of 3 He and CNO Isotopes in the Envelopes of Low‐Mass Red Giants
The Astrophysical Journal, 2008
Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3 He(3 He,2p) 4 He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3 He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism operates rapidly once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0 M develop 12 C/ 13 C ratios of 14.5 ± 1.5, while Pop II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 M , this mechanism also destroys 90% to 95% of the 3 He produced on the main sequence.
The Astrophysical Journal, 2011
We reanalyze the problem of Li abundances in red giants of nearly solar metallicity. After an outline of the problems affecting our knowledge of the Li content in low-mass stars (M ≤ 3M ⊙ ), we discuss deep-mixing models for the RGB stages suitable to account for the observed trends and for the correlated variations of the carbon isotope ratio; we find that Li destruction in these phases is limited to masses below about 2.3 M ⊙ . Subsequently, we concentrate on the final stages of evolution for both O-rich and C-rich AGB stars. Here, the constraints on extra-mixing phenomena previously derived from heavier nuclei (from C to Al), coupled to recent updates in stellar structure models (including both the input physics and the set of reaction rates used), are suitable to account for the observations of Li abundances below A(Li) ≡ log ǫ(Li) ≃ 1.5 (and sometimes more). Also their relations with other nucleosynthesis signatures of AGB phases (like the abundance of F, the C/O and 12 C/ 13 C ratios) can be explained.
ON THE NEED FOR DEEP-MIXING IN ASYMPTOTIC GIANT BRANCH STARS OF LOW MASS
The Astrophysical Journal, 2010
The photospheres of low-mass red giants show CNO isotopic abundances that are not satisfactorily accounted for by canonical stellar models. The same is true for the measurements of these isotopes and of the 26 Al/ 27 Al ratio in presolar grains of circumstellar origin. Non-convective mixing, occurring during both red giant branch (RGB) and asymptotic giant branch (AGB) stages, is the explanation commonly invoked to account for the above evidence. Recently, the need for such mixing phenomena on the AGB was questioned, and chemical anomalies usually attributed to them were suggested to be formed in earlier phases. We have therefore re-calculated extra-mixing effects in low-mass stars for both the RGB and AGB stages, in order to verify the above claims. Our results contradict them; we actually confirm that slow transport below the convective envelope occurs also on the AGB. This is required primarily by the oxygen isotopic mix and the 26 Al content of presolar oxide grains. Other pieces of evidence exist, in particular from the isotopic ratios of carbon stars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC grains of AGB origin. We further show that, when extra-mixing occurs in the RGB phases of Population I stars above about 1.2 M , this consumes 3 He in the envelope, probably preventing the occurrence of thermohaline diffusion on the AGB. Therefore, we argue that other extra-mixing mechanisms should be active in those final evolutionary phases.
Canonical Extra Mixing in Low‐Mass Red Giants
The Astrophysical Journal, 2003
We have used the latest observational data on the evolutionary variations of the surface chemical composition in low-mass metal-poor stars, both in the field and in globular clusters, to constrain the basic properties of extra mixing in upper red giant branch (RGB) stars. Two different models of extra mixing have been incorporated into a stellar evolution code: a parametrical diffusion model and a model with rotationinduced turbulent diffusion. Application of the first model to the interpretation of the observed variations of the surface abundances of Li, C, and N and of the isotopic ratio 12 C/ 13 C in field stars has revealed that, for the majority of upper RGB stars, the depth and rate of extra mixing do not appear to vary appreciably from star to star. Furthermore, comparisons of our calculations with the results obtained by other authors show that at least the extra mixing depth does not seem to depend strongly on metallicity. Therefore, we propose to call this universal nonconvective mixing process '' canonical extra mixing.'' We also put forward the hypothesis that some of the upper RGB stars may be experiencing '' enhanced extra mixing,'' which is much faster (by a factor of 100)andsomewhatdeeperthancanonicalextramixing.ThiscouldexplainthephenomenonofLi−richgiants.EnhancedextramixingcouldalsocontributetotheO−NaandMg−Alanticorrelationsthatareseeninsomeglobularclusterredgiants.ApossiblemechanismofextramixinginupperRGBstarsmaybeturbulentdiffusionor/andmeridionalcirculationinducedbyrotation.Inthiscase,enhancedextramixingrequiresrotationalvelocitiesthatare100) and somewhat deeper than canonical extra mixing. This could explain the phenomenon of Li-rich giants. Enhanced extra mixing could also contribute to the O-Na and Mg-Al anticorrelations that are seen in some globular cluster red giants. A possible mechanism of extra mixing in upper RGB stars may be turbulent diffusion or/and meridional circulation induced by rotation. In this case, enhanced extra mixing requires rotational velocities that are %10 times as fast as those that are sufficient for the occurrence of canonical extra mixing. Observations do not exclude this possibility because (1) the dispersion in the surface rotational velocities of field Li-rich giants span a range of a factor of 100)andsomewhatdeeperthancanonicalextramixing.ThiscouldexplainthephenomenonofLi−richgiants.EnhancedextramixingcouldalsocontributetotheO−NaandMg−Alanticorrelationsthatareseeninsomeglobularclusterredgiants.ApossiblemechanismofextramixinginupperRGBstarsmaybeturbulentdiffusionor/andmeridionalcirculationinducedbyrotation.Inthiscase,enhancedextramixingrequiresrotationalvelocitiesthatare10 and (2) the extremely fast rotation of blue horizontal branch stars in globular clusters may require that their RGB precursors had been spun up appreciably by an external source. Star-to-star abundance variations in globular clusters may well have been produced as the result of both evolutionary and primordial processes. In the primordial scenario, the nuclearly processed material that is accreted by low-mass main-sequence stars may have originated primarily in earlier generations of massive asymptotic giant branch stars that had undergone hot bottom burning of their envelopes and partly in mass-losing upper RGB stars that had been just a bit more massive than the present-day main-sequence turnoff stars and had experienced extra mixing in the past.
Evolution and surface abundances of red giants experiencing deep mixing
Astronomy and Astrophysics
We have calculated the evolution of low metallicity red giant stars under the assumption of deep mixing between the convective envelope and the hydrogen burning shell. We find that the extent of the observed abundance anomalies, and in particular the universal O-Na anticorrelation, can be totally explained by mixing which does not lead to significant helium enrichment of the envelope. On the other hand, models with extremely deep mixing and strong helium enrichment predict anomalies of sodium and oxygen, which are much larger than the observed ones. This latter result depends solely on the nucleosynthesis inside the hydrogen burning shell, but not on the details of the mixing descriptions. These, however, influence the evolution of surface abundances with brightness, which we compare with the limited observational material available. Our models allow, nevertheless, to infer details on the depth and speed of the mixing process in several clusters. Models with strong helium enrichment...
DEEP MIXING IN EVOLVED STARS. I. THE EFFECT OF REACTION RATE REVISIONS FROM C TO Al
The Astrophysical Journal, 2011
We present computations of nucleosynthesis in low-mass red-giant-branch and asymptotic-giant-branch stars of Population I experiencing extended mixing. We adopt the updated version of the FRANEC evolutionary model, a new postprocess code for non-convective mixing and the most recent revisions for solar abundances. In this framework, we discuss the effects of recent improvements in relevant reaction rates for proton captures on intermediate-mass nuclei (from carbon to aluminum). For each nucleus we briefly discuss the new choices and their motivations. The calculations are then performed on the basis of a parameterized circulation, where the effects of the new nuclear inputs are best compared to previous works. We find that the new rates (and notably the one for the 14 N(p,γ) 15 O reaction) imply considerable modifications in the composition of post-main sequence stars. In particular, the slight temperature changes due to the reduced efficiency of proton captures on 14 N induce abundance variations at the first dredge up (especially for 17 O, whose equilibrium ratio to 16 O is very sensitive to the temperature). In this new scenario presolar oxide grains of AGB origin turn out to be produced almost exclusively by very-low mass stars (M ≤ 1.5 − 1.7M ⊙), never becoming C-rich. The whole population of grains with 18 O/ 16 O below 0.0015 (the limit permitted by first dredge up) is now explained. Also, there is now no forbidden area for very low values of 17 O/ 16 O (below 0.0005), contrary to previous findings. A rather shallow type of transport seems to be sufficient for the CNO changes in RGB stages. Both thermohaline diffusion and magnetic-buoyancy-induced mixing might provide a suitable physical mechanism for this. Thermohaline mixing is in any case certainly inadequate to account for the production of 26 Al on the AGB. Other transport mechanisms must therefore be at play. In general, observational constraints from RGB and-3-AGB stars, as well as from presolar grains, are well reproduced by our approach. An exception remains the nitrogen isotopic ratio in mainstream SiC grains. For the low values measured in them (i.e. for 14 N/ 15 N ≤ 2000) we have no explanation. Actually, for the several grains with subsolar nitrogen isotopic ratios no known stellar process acting in low mass stars can provide a clue. This might be an evidence that some form of contamination from cosmic ray spallation occurs in the interstellar medium, adding fresh 15 N to the grains.
We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ −2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellarparameter differences between groups of stars, are in good agreement with broad-band and Strömgren photometry calibrated on the infrared-flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 31% less abundant in the turnoff-point stars than in the red giants. An abundance difference in lithium is seen between the turnoff-point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, ...